1. Field of the Invention
The present invention relates to a laminated deodorant filter medium and a method of making the same. In particular, it relates to a laminated deodorant filter medium, which is excellent in deodorizing performances and processability, used for purifying a fluid contaminated with odor components by filtration, for example, and a method of making the same.
2. Related Background Art
Recently, awareness of living environment has enhanced remarkably. In particular, there is a great interest in odor. Accordingly, it has conventionally been required that not only harmful materials but also unpleasant odoriferous materials be eliminated. In order to remove these odor components, various types of deodorant filter media have been proposed. Most of such deodorant filter media comprise a gas-permeable sheet member, and particles made of activated carbon or various kinds of chemical deodorants (hereinafter generically referred to as deodorant particle), for example, firmly attached to and carried by the sheet member.
Widely employed as a conventionally known technique for firmly attaching particles are processes using various kinds of binders to firmly attach deodorant particles to a base fabric made of a nonwoven or the like so that they are carried by the base fabric. Most of these processes have shortcomings in that, since the surface of deodorant particle is covered with the binder, the performances inherent in the particle may be deteriorated.
On the other hand, for example, in the technique proposed by Japanese Patent Application Laid-Open No. 8-266845, an emulsion type acrylic adhesive is applied beforehand to a net having a mesh size of 1 mm to 10 mm, and deodorant particles (adsorbent particles) are firmly attached to the net by way of the adhesive. In this technique, as disclosed in the above-mentioned publication, the net having a mesh size of 1 mm to 10 mm is coated with the adhesive by use of spray, coater, or the like. Accordingly, even if a net having a mesh size of 100 .mu.m or less, for example, is chosen in order to achieve a desired deodorizing efficiency, the aperture portion of the net may be blocked at the adhesive application step, whereby the resulting filter medium may yield a higher pressure loss.
Known as another technique for firmly attaching particles is a process in which an adhesive made of a particulate thermoplastic resin and deodorant particles are mixed together beforehand, and the resulting mixture is applied to a sheet-shaped flexible support material, so as to yield a filter medium. While an adhesive having a particle size smaller than that of the deodorant particle is usually employed in thus configured deodorant filter medium, the deodorant particles can be mutually bonded together point by point in a three-dimensional fashion by way of binder particles, so as to constitute a three-dimensional structure with a predetermined thickness. Accordingly, as compared with the technique disclosed in the previously mentioned publication, a deodorant filter medium comprising deodorant particles with a higher density can be realized.
As explained in the foregoing, in the techniques using a particulate adhesive, deodorant particles and binder particles are mixed beforehand, and the resulting mixture is disposed at and firmly attached to a support material. Here, when one deodorant particle is taken into consideration, the degree of attachment thereof can be controlled when the particle size of binder particles and their relative composition ratio with respect to the deodorant particle are selected. The binder particle, however, merely holds the deodorant particles at their discrete points. Accordingly, as is well-known, the mechanical strength of binder particles is not high enough to effect bending processing of a filter element in order to materialize a highly efficient deodorant filter medium. Hence, in order to prevent the deodorant particles from dropping off, it has been necessary to limit the designing upon the bending processing. Also, in this point-bonding technique, the adhesive is needed to be hot-melted in order to attain sufficient attachment. Upon heat treatment, however, the adhesive is likely to fluidize so as to localize on the lower side in the direction of gravity. Consequently, it has been difficult to set the heat-treatment condition, whereby the pressure loss caused by the adhesive is likely to increase, or the deodorant particles may be covered with the fluidized adhesive.
Meanwhile, there are various kinds of deodorants used as deodorant particles. Activated carbon, which is the most prevailing, exhibits a relatively low specificity to odoriferous materials and has a function of physical adsorption. (Such a kind of deodorant particles will hereinafter be referred to as physically adsorbing particle.) In addition, with odoriferous materials being classified into those of acidic odor and alkaline odor, deodorant particles having chemical functions of capturing these odors by use of their respective charge states (hereinafter respectively referred to as acidic odor deodorant particle and alkaline odor deodorant particle according to odoriferous materials which can be neutralized thereby) have been known.
Known as a deodorant filter medium comprising two or more kinds of deodorant particles having different functions as such is a technique disclosed in Japanese Utility Model Application Laid-Open No. 4-30025, for example. This publication discloses a filter having a pleated structure in which a net-shaped porous sheet and another porous sheet overlap each other, with one of the sheets having a deodorizing function caused by a chemical reaction or catalytic action. It also discloses a filter having a pleated structure in which a net-shaped porous sheet and another porous sheet overlap each other, with one of the sheets containing a deodorizing adsorbent, while the other having a deodorizing function caused by a chemical reaction or catalytic action.
The inventors have studied, in detail, deodorant filter media satisfying both pressure loss and deodorizing efficiency by using two or more kinds of deodorant particles having different functions. For example, for preparing a sheet-shaped deodorant filter medium by use of two kinds of deodorant particles, the simplest technique comprises the steps of uniformly mixing desired deodorant particles and then firmly attaching them to a gas-permeable cloth. When the pressure loss as the deodorant filter medium is taken into consideration, it is necessary that the deodorant particles be designed to have a predetermined particle size or greater. It has been found as a result of these empirical studies that, as can be understood from FIG. 4 that shows the cross section of a deodorant filter medium by its schematic cross section, the probability of first deodorant particles 1, which have a deodorizing function with respect to an odoriferous material, coming into contact with a flow of the odoriferous material indicated by arrow A substantially coincides with the ratio of the area occupied by the first deodorant particles 1 to the filter medium area (area through which the gas to be filtered can pass) of the filter medium 5. Accordingly, it has been found that, in the portion of the filter medium 5 occupied by second deodorant particles 3, the odoriferous material exhibits a phenomenon similar to leaking, thus making it difficult to efficiently effect deodorization in a structure in which two kinds of different deodorant particles are firmly attached into a single layer. In effect, it is quite difficult for two different kinds of deodorant particles to be firmly attached to and carried by a cloth such that one kind of the particles and the other kind of particles are completely adjacent to each other on the cloth surface.
Also, as described in the above-mentioned Japanese Utility Model Application Laid-Open No. 4-30025 as well, of materials having a chemically deodorizing function, those exhibiting this function upon neutralizing reactions have been known to include a number of deodorant materials which are deactivated when coming into contact with mutually-reacting materials such as acid and base. Accordingly, in a process where such deodorant particles which are likely to be deactivated come into contact with each other, the deodorizing performances to be exhibited may be inhibited.
For designing a deodorant filter medium with a pressure loss as low as possible, it will be effective if different kinds of deodorant particles are laminated layer by layer. Nevertheless, there has been no specific technique known for preventing the deodorant particles from being deactivated upon coming into contact with each other between the layers.